Some patients with idiopathic type 1 diabetes have a fulminant disorder characterized by the absence of insulitis and of diabetes-related antibodies, a remarkably abrupt onset and high serum pancreatic enzyme concentrations. This is referred to as fulminant type 1 diabetes. Cardiopulmonary disorders are rarely observed around the onset of fulminant type 1 diabetes. A 51-year-old woman suffering from nausea and vomiting was transferred to our hospital. Laboratory findings revealed high blood glucose level and the evidence of diabetic ketoacidosis, but the serum HbA1c was normal nevertheless. The low level of plasma C-peptide indicated the loss of endogenous insulin secretion. The patient satisfied the criteria for the diagnosis of fulminant type 1 diabetes. Electrocardiogram (ECG) revealed nonspecific ST-T-wave abnormalities. Transthoracic echocardiogram demonstrated that she had severe pulmonary hypertension and minimal pericardial effusion. In a week, pulmonary hypertension improved to mild degree without specific treatment. Acute myocarditis was suspected based upon flulike symptoms, nonspecific ST-T-wave abnormalities, minimal pericardial effusion and asymptomatic pulmonary hypertension. We considered it worthwhile reporting this case because fulminant type 1 diabetes with acute myocarditis has never been published yet.